A radar apparatus, which emits a transmission wave to a periphery and detects a reflected wave through a transparent window, is for example employed to a laser radar apparatus for a vehicle to detect a distance between the vehicle and an object existing around the vehicle. In such a laser radar apparatus, if dirt such as mud, oil, raindrops, snow or insects adheres to the window, detection accuracy is likely to deteriorate.
For example, Japanese Unexamined Patent Publication No. 9-211108 discloses a scanning-type laser radar apparatus capable of detecting the degree of dirt adhered to the transparent window. Specifically, a laser light is reflected and scattered by a scattering plate provided at one of left and right ends of the apparatus. A laser light, which is reflected and scattered into the inside from a transparent window by dirt adhered to an outside of the transparent window, is received by a photodetector. When the amount of the received light is more than a predetermined threshold, it is determined that the amount of dirt adhered to the transparent window is more than an allowable amount. Although the apparatus can detect the dirt, it is necessary to remove the dirt every time the dirt is detected.